1. Field of the Invention
The present invention relates to biocontrol of replant disease of tree fruits. More particularly, the invention relates to a unique strain of Pseudomonas putida, methods of using the strain to control replant disease, and agricultural compositions containing the bacterium, which are useful in such methods.
2. Description of the Art
Replant diseases of various crops have been recognized since the late 17th century. In apple, replant disease is widespread and has been documented in all of the major fruit-growing regions of the world. Replant disease of apple is most prominent at sites where replanting occurs after the removal of very old fruit trees, but anecdotal evidence suggests that the disease may also be important in nurseries where previous plantings of apple were grown for one to two years (A. L. Jones and H. S. Aldwinckle, Compendium of Apple and Pear Diseases, APS Press, St. Paul, Minn. (1990)), and development of a soil microflora conducive to the development of replant disease was observed within three years of orchard establishment at a previously uncultivated site in Washington state (M. Mazzola, Phytopathology 87:S63 (1997) (abstract)). Uneven growth of young apple trees is a common indication of apple replant disease, but when severe disease pressure is encountered, poor growth or mortality may be exhibited by a majority of trees in the orchard. Symptoms of apple replant disease include severe stunting, shortened internodes, rosetted leaves, and reduced productivity, and trees possess small root systems with an abundance of fibrous roots, many of which are nonfunctional due to decay (F. L. Caruso et al., Canadian Journal of Botany 67:742-749 (1989); H. Hoestra, Ph.D. Thesis, Meded. Landbouwhogesch, Wageningen (1968); and B. M. Savory, Ph.D. Thesis, University of London, London (1966)).
The primary tree fruit production region in North America resides in the Western United States. Apples are grown on over 200,000 acres in this region and account for 65% of the total U.S. production. In Washington state, apples are grown on approximately 180,000 acres, are the source of nearly 50% of the total United States production, and typically vie with wheat as the major agricultural commodity in the state. In 1996, the Washington apple crop generated over one billion dollars in farm income.
Replant disease is often the major impediment to the establishment of an economically viable orchard on a site previously cropped to apple. In Washington state, failure to control apple replant disease typically will result in a $40,000 per acre reduction in gross returns over a ten year period. In addition to tree replacement costs incurred on sites exhibiting severe replant symptoms, trees affected by the disease begin bearing fruit 2-3 years later than normal and fail to attain yields comparable to those obtained in orchards free of the disease. Apple replant disease is becoming an increasingly important problem as orchard rotations are shortened and availability of land suitable for orchard establishment, but not previously planted to apple, becomes limited. Approximately 10,000 acres of apple are replanted each year in the state of Washington alone, and this figure is continuing to rise. Increased frequency of orchard replacement also has resulted from development of markets for new apple varieties, adoption of intensive orchard practices which utilize compact tree spacing resulting in earlier, higher returns/acre, and adoption of dwarfing rootstocks, which have a shorter economic life expectancy than the seedling rootstocks which they replace (A. B. Peterson, Good Fruit Grower, Wenatchee, Wash. (1992)).
Control of replant problems of tree fruits in the United States has traditionally been achieved through the use of soil fumigants, including methyl bromide. Application costs for pre-plant fumigation using methyl bromide/chloropicrin average $600/acre and associated costs are higher for metam sodium application. However, the parties to the United Nations' Montreal Protocol on Substances that Deplete the Ozone Layer have agreed to a stepped phaseout in the use of methyl bromide prior to elimination of its use by 2010. The United States Congress, by way of the Clean Air Act, has prohibited the production and importation of methyl bromide after Jan. 1, 2001. Other broad spectrum biocides, including metam sodium, telone, and chloropicrin, have been proposed as replacements to methyl bromide; the continued use of each of these materials faces potential obstacles as problems ranging from groundwater contamination to health concerns are raised. For example, while pre-plant application of metam sodium has been utilized by some growers in Washington state, this fumigant has been identified for possible regulatory action under section 210 of the Food Quality Protection Act that amends the Federal Insecticide, Fungicide and Rodenticide Act, and the long-term availability of this material remains in doubt. In addition to potential regulatory restrictions, there are several other disadvantages inherent to the continued use of broad spectrum biocides, including difficulty in application, high cost, and the potential hazards to human health. In addition, at present the expanding organic tree fruit industry has no alternative but preplant fumigation prior to orchard renovation (D. Granatstein, Good Fruit Grower pp. 14-15 (1997)). Orchardists must accept the economic loss associated with the three year period after fumigation in which fruit cannot be marketed as organic. Thus, alternative approaches to the use of preplant soil fumigants in general for control of soilborne diseases are needed to ensure the continued productivity of orchard and fruit tree nursery operations.
Replant disease of apple has been studies by numerous investigators for many years, but the etiology of the disease remains to be clearly defined. Replant disease of apple has been attributed to a variety of biotic and abiotic factors, but the fact that other fruit tree species planted in the same soil grow normally, and that soil pasteurization or fumigation dramatically improve plant growth provide conclusive evidence that this disease is primarily a biological phenomenon rather than the result of abiotic factors. Although soil arsenic residues were implicated as a potential cause of replant disease, Covey et al. (Phytopathology 71:712-715 (1981)) observed no reduction in the growth of apple in soils amended with arsenic at concentrations of up to 200 ppm. Likewise, Merwin et al. (Chemosphere 29:1361-1367 (1994)) found no correlation between arsenic concentration and the growth of apple seedlings in old orchard soils in New York.
Previous studies have suggested that the disease is of complex etiology and the factors implicated as causal agents and predisposing factors have been reported to vary between orchard sites. Numerous soil- and plant-associated microorganisms have been implicated as potential causal agents of apple replant disease. The lesion nematode (Pratylenchus spp.) was thought to have a major role in apple replant disease in the eastern United States (Jaffee et al., Phytopathology 72:247-251 (1982) and Mai et al., Plant Disease 65:859-864 (1981)), British Columbia (Utkhede et al., Plant and Soil 139:1-6 (1992)) and Australia (Dullahide et al., Aust. J. Exp. Agr. 34:1177-1182 (1994)). However, the data presented in these studies are far from decisive and, in some instances, are in direct conflict with the conclusion that the lesion nematode has a role in this disease phenomenon. For instance, Dullahide et al., supra, concluded that the lesion nematode was an important component of the disease complex in Australia, yet elimination of this nematode had no significant effect on apple growth in any of eight replant soils surveyed, even in soils where pasteurization significantly enhanced apple growth. Utkhede et al., 1992, supra, also concluded that P. penetrans contributed to the development of apple replant disease, yet in greenhouse studies there was no correlation between root populations of this nematode and disease severity. In studies conducted by Jaffee et al., Phytopathology 72:247-251 (1982), populations of P. penetrans did not increase over a six week period of apple seedling growth, and a significant reduction in growth of apple seedlings was only obtained when nematode populations were artificially augmented.
Several studies have suggested a role for soilborne fungi in the etiology of apple replant disease. In certain replant soils, the fungicides captan and mancozeb (MANZATE 200) were as effective as soil pasteurization or fumigation with methyl bromide in controlling apple replant disease (Slykhuis et al., Can. J. Plant Pathol. 7:294-301 (1985)). Species of Pythium (Braun, Can. J. Plant Pathol. 17:336-341 (1995); Caruso et al., Can. J. Bot. 67:742-749 (1989); Jaffee et al., Plant Disease 66:942-944 (1982); Sewell, Ann. Appl. Biol. 97:31-42 (1981)) and Cylindrocarpon (Braun, supra, and Jaffee et al., Plant Disease 66:942-944 (1982)) have repeatedly been implicated as causal agents of apple replant disease. A number of other fungi, including Phytophthora spp., Armillaria mellea (Sutton et al., Plant Disease 65:330-332 (1981)), Peniophora sacrata (Taylor et al., Can. J. Plant Pathol. 434:263-265 (1970)), Mortierealla sp., Torulomyces lagena and Trichoderma hamatum (Utkhede et al., 1992, supra), have been implicated in disease development on a site-specific basis. Other studies have suggested the possible involvement of fluorescent pseudomonads and actinomycetes as potential causal agents of replant disease (Bunt et al., Meded. Fac. Landbouwwet. Rijksuniv. Gent. 38:1381-1385 (1973)) and Wescott et al., Phytopathology 77:1071-1077 (1987)), however the data implicating these organisms in the disease etiology is tenuous.
Recently, we conducted studies to elucidate the role of different soil microorganisms in the development of apple replant disease at multiple sites in Washington state (Mazzola, Phytopathology 87:582-587 (1997); Mazzola, Phytopathology 87:S63 (1997) (Abstracts)). The results of these studies demonstrated that fungi are the dominant causal agents of replant disease in Washington state, and that the relative importance of the individual pathogens of species from the genera Cylindrocarpon, Phytophthora, Pythium and Rhizoctonia in disease development varies among sites.
In large part, biological and cultural measures for the control of replant disease have failed to be effective under field conditions and have not been adopted by commercial growers. With the exception of monitoring populations of P. penetrans, there is a general absence of studies which have determined the implications of alternative control measures on the populations or activity of putative causal agents of apple replant disease under field conditions.